The present invention concerns an evaporated fuel treatment apparatus for an internal combustion engine which releases evaporated fuel generated inside the fuel tank into the intake manifold of the internal combustion engine. More concretely, the present invention concerns an evaporated fuel treatment apparatus for an internal combustion engine which makes it possible to ascertain the presence or absence of leakage in an evaporated fuel discharge prevention system extending from the fuel tank to the engine intake system.
A method for judging the presence or absence of leakage in a tank system is described in Japanese Patent Application Kokai No. Hei 7-83125. In this method, the pressure in an evaporated fuel discharge prevention system is lowered to a specified pressure; next, with the target pressure reduction value of the pressure in the fuel tank alternately set at an upper-limit value and a lower-limit value, a feedback pressure reduction process which gradually causes the pressure in the fuel tank to converge on the target pressure reduction value is performed, and the amount of pressure shift in the fuel tank per unit time is calculated (leak down checking mode). In order to eliminate the effect of vapor on the judgement results, the amount of pressure shift per unit time caused by the evaporated fuel is calculated as a correction value. Judgement of the presence or absence of leakage in the tank system is accomplished on the basis of a value obtained by subtracting the value produced by multiplying the amount of pressure shift calculated in the correction checking mode by a coefficient from the amount of pressure shift calculated in the above-mentioned leak down checking mode. If this value is equal to or less than a specified value, it is judged that the tank system is normal, with no leakage; on the other hand, if this value exceeds the specified value, it is judged that there is leakage in the tank system.
However, in cases where the presence or absence of leakage in a tank system is judged by performing such a leak check and correction check, an accurate correction cannot be performed if there is a great difference between the driving conditions during the correction check and the driving conditions during the leak check. For example, in a case where the vehicle is in stable cruise operation during the correction check, but conditions of acceleration are included in the leak check, the tank system may be judged to be normal even if leakage is present, since the rise in the internal pressure of the fuel tank is reduced during acceleration due to consumption by the engine.
However, the rise in the internal pressure of the fuel tank is accelerated during vehicle deceleration due to factors such as reduced fuel consumption, etc., and the rise in the internal pressure of the fuel tank is retarded (i.e., the internal pressure shifts in the negative pressure direction) during vehicle acceleration due to factors such as increased fuel consumption, etc.
For example, in order to detect leakage caused by a very small hole such as a hole with a diameter of 0.5 mm, a considerable length of time ranging from 30 seconds to 60 seconds is required for the correction check and leak check, so that the driving conditions of the vehicle may vary greatly during the correction check and leak check. In such cases, the difference in driving conditions during the correction check and leak check has an effect on the internal pressure of the fuel tank, and adversely affects the precision with which the above-mentioned judgement of the presence or absence of leakage is made.
Thus, the present invention is directed to overcoming the above-mentioned problems, and to improve the reliability of the above-mentioned judgement.
An evaporated fuel treatment apparatus is provided, which comprises correction checking means which detects the magnitude or rate of change in the internal pressure of the fuel tank when the fuel tank is closed after being placed under atmospheric pressure, a leakage checking means which detects the magnitude or rate of change in the internal pressure of the fuel tank when the fuel tank is closed after being placed under a negative pressure. Judgement means is included which judges the presence or absence of leakage in the fuel tank on the basis of the detection results obtained by the leakage checking means and the correction checking means. The apparatus further includes calculating means which calculates the amount of fuel consumption at the time of the correction checking and the amount of fuel consumption at the time of leakage checking. Judgement prohibiting means prohibits a judgement of the presence or absence of leakage from being made by the judgement means in cases where the calculated amounts of fuel consumption are substantially different.
The expression that the calculated amounts of fuel consumption are substantially different is defined to mean that the difference is such that it affects the judgement of presence or absence of leakage. Specifically, it means a difference that is greater than a predetermined magnitude, which is determined by experiments or simulation.
According to the invention, because a judgement of the presence or absence of leakage in the tank system is prevented from being performed in cases where such a judgement might be inaccurate, erroneous judgement is avoided and therefore, reliability of the judgement is enhanced.